The Cassini Environmental Impact Statement, Chapter 2 (page 53–54) describes the status of Concentrated Solar Energy in space:
A 1981 conceptual study of the system impacts of using a concentrated solar array (CSA) on the Galileo spacecraft, launched in 1989 to Jupiter and powered by two RTGs, concluded that such an effort could be performed but would require an "extensive development effort" and that "..the severe environmental constraints and the embryonic state of CSA development indicates that CSA will not displace the RTG on the Galileo mission" (Rockey et al. 1981).
Concentrators have not been demonstrated in space, and a number of significant technical problems would have to be solved before a concentrator could be considered feasible for space missions, such as Cassini. The problems include how to regulate the concentrator's temperature for acceptable performance as the spacecraft traverses a Sun-to-spacecraft range from 0.63 AU to 9.3 AU; how to predict the behavior of the optics over the mission lifetime, because small changes in the concentrator condition (e.g., yellowing, aging, and sagging) can lead to significant power losses; and how to improve the alignment of the concentrator elements due to the dependence of the concentrator's power-generating ability on the Sun's incident angle. In addition, concentrator performance depends on clear, unobscured optics, and estimating the buildup of interstellar (and Saturnian) dust on the optics would be difficult. Moreover, vibration testing of any concentrator array would have to be performed to verify post-launch optical alignments and operating characteristics in zero gravity environments. The size of the concentrator arrays that would be needed for the exploration of the Saturnian system would not easily integrate into the Titan IV (SRMU)/Centaur and would not satisfy the launch mass constraints. Furthermore, it is not clear that concentrator arrays would provide any advantage over planar arrays for this mission (JPL 1994a).
1981 is a long time ago, and there might have been plenty of "extensive development effort". What is the current status of concentrated solar arrays in space? Have they ever been used, or studied more recently than in the cited study?